Answer
One way would be to apply a linear gradient on the stroke. Create a circle, and stroke it with a gradient pattern. Make sure the gradient has the stops like in the table and picture below. The gradient might not look as smooth in the transitions, but that’s all that comes to mind.
+------------+---------------+----------+
| Color Stop | RGB | Location |
|------------+---------------+----------|
| 1 | (255, 0, 255) | 0.00% |
| 2 | (0, 0, 255) | 16.67% |
| 3 | (0, 255, 255) | 33.33% |
| 4 | (0, 255, 0) | 50.00% |
| 5 | (255, 255, 0) | 66.65% |
| 6 | (255, 0, 0) | 83.33% |
| 7 | (255, 0, 255) | 100.00% |
+---------------------------------------+
Attribution
Source : Link , Question Author : dcRay , Answer Author : Raffi